Seismic images can reveal remarkable pictures of the earth's
subsurface geology. Such
pictures can be used to understand and identify faults at deep depths, and
therefore help us identify the location of oil and gas deposits.
Figure 1.2
shows
some seismic pictures of faults taken by optical and seismic cameras.
The explorationist is keenly interested in such faults
because the lighter-than-water
oil+gas will flow along a layer towards a topographic high
until it is stopped by some permeability barrier such as a fault.
There it accumulates in large reservoirs, waiting to be found by
the explorationist with the most capable seismic camera.

A fault is not a sufficient condition
for a hydrocarbon reservoir, so
one must look for additional hydrocarbon clues in the seismic image.
Such clues can sometimes be found in the brightening
of the reflection amplitude.
Gassy marine sandstones will typically reflect more energy than
non-gas sands, so a gas can reveal
its presence by
a large amplitude bright spot in the seismic section.
A bright spot example from the Gulf of Mexico
is shown in Figure 3 where the gas-sand reflection is much "brighter"
than the surrounding reflections. This brite
spot technology represented
one of the technological breakthroughs in the 1970's
that led to a significant increase in the hydrocarbon discovery
rate
in the Gulf of Mexico and other marine environments.

Figure 1.3:
Brite spot, or large seismic amplitude
in the seismic section indicating a large reservoir of gas.
Vertical axis is in seconds, and 1 second indicates a depth of about 5,000
feet (courtesy of Conoco).

Another breakthrough in seismic exploration,
3-D reflection experiments, came about with the
advent of fast computers in the mid-1980's.
Instead of just placing geophones and sources along the same line, 3-D
seismic experiments distribute
geophones and sources on a large 2-D patch of ground.
A large 2-D grid of
geophones is activated for each source location so that
3-D seismic scattering from out-of-the XZ plane
could be properly accounted for to reveal geologic complexity in
unprecedented detail. An example is given Figure 1.4, where
a 3-D reflectivity cube of data is sliced horizontally
to reveal a meandering river channel at a depth of
more than 16,000 feet!. River channels can make
oil geologists gush with excitement because they
contain highly porous sands that can quickly sop up vast
pools of migrating hydrocarbons.

Figure 1.4:
Seismic 3-D image cube XYZ space showing
a horizontal XY slice of a meandering river
channel at a depth of more than 4 miles. The color variations correspond
to variations in reflectivity. This
picture might look similar to an airplane view of the ground if you flew over
the Mississippi River at a 20,000' elevation. The ability
to "seismically see"
within the earth to depths of many miles is one of the great
technological achievements of the 20th century
(modified from Dave Lumley's homepage)!